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I I I I I I, I I I I I I I I I I I I I year flow. The drainage structures under 1-70, Cole Boulevard and Eldridge Street, and Cole Boulevard downstream are all sized with adequate capacity for the 100-year flows without overtopping. Adequate erosion protection has been provided in the channel at most locations. crossings. In addition, the maintenance level improvements which have been constructed have helped stabilize the channel eliminating severe damages from erosion. However, all the improvements done in this reach have not significantly enlarged the channel capacity and the damage potential to residential properties is still relatively high. The concrete box culvert under Youngfield Street is quite undersized and, because of the high road embankment, causes a backwater effect and overtopping of the street during greater than about the 50-year flood. The three small detention ponds in the lower portion of Denver West serve primarily to contain local runoff from the development. Sediment accumulation in the ponds is a continuing maintenance concern requiring periodic removal and disposal by Denver West owners. Damages in this reach are expected to be minor and consist mostly of debris cleanup and removal. L.l Sta. 163+55 to 191 +60: Sheets 11-12 L-2 Sta. 191 +60 to 223+00: Sheets 10-11 Downstream of Youngfield Street the channel continues through single-family residential property. The channel in this reach is mostly unimproved, although several property owners have attempted improvements of their own. In this reach, as in Reach L-2 just upstream, several homes lie either directly in or near the limits of the 100-year floodplain. The floodplain is contained between properties which are immediately adjacent to the drainageway. The previous culvert at 20th Avenue was undersized and had a history of flooding problems. Two residential properties located just upstream of 20th Avenue on the east have repeatedly been damaged. A recently completed (November, t992) replacement structure at 20th Avenue will help to alleviate the flooding problems that were associated with the previous undersized culvert. The enlarged opening area combined with the concrete f100dwall along the southeast edge of the drainageway reduce the potential for property damage just upstream of 20th Avenue. The outlet of the lower Denver West Pond is the upstream limit of this reach. Approximately 250 feet downstream of the pond, Lena Gulch crosses the Agricultural Irrigation Ditch. Like the Welch Ditch in Reach L-4, the Agricultural Ditch crosses over the Lena Gulch channel in a rectangular concrete flume. The opening under the flume only has capacity for less than the 1 O-year flows without overtopping the flume. Downstream of the Agricultural Ditch, the Lena Gulch channel enters a single- family residential development. The channel through this reach only has capacity for about the Z-year flow. Several homes adjacent to the drainageway are located either directly in, or near the edge of the 1 DO-year floodplain. Past removal of the culvert crossings at Alkire Street, Alkire Court, and Arbutus Street have mostly eliminated the constrictions which previously existed at those 17